RV-12 Drag & Performance Guesses

RV12 Drag curve

Goal: Develop an understanding of RV12 performance by estimating the RV12’s Drag curves

Disclaimer: I know enough about aerodynamics to know that I do not understand it.

Intent of this post: To start a discussion about these fundamental characteristics of the RV12.

Assumptions: Sea Level, standard day, aircraft at maximum gross weight, flaps up

Start: RV6a Drag information from the Café Foundation APR (aircraft performance report) of the RV6a. This test can be found at: http://cafefoundation.org/v2/pdf_cafe_apr/RV-6A Final APR.pdf

From the RV6a drag graph in the report, the coordinates of the induced and parasitic drag data points were obtained. This data was then entered into Excel and curves fit. Induced drag is supposed to equal a constant divided by the square of the airspeed. Parasitic drag is supposed to equal a constant times the square of the airspeed (from Aerodynamics for Naval Aviators). My goal was to find the constants that resulted in the best fit. These are the formula of the best fit lines.

Induced Drag (pounds) = 764669.98 / CAS(mph)^2
Parasitic Drag (pounds) = 0.00594791 * CAS(mph)^2

Total Drag is the sum of the Induced and Parasitic drag values at each of the airspeeds.

Creating the RV12 Drag Curve Estimates:

Theses estimates were created following these (possibly misguided) ideas:
Began with the induced curve
1) Shifted the stall speed by 3 mph (RV6a reported stall 55 mph cas, RV12 is 52). Since both share essentially the same airfoil (RV6a 23013.5, RV12 23014) they should therefore stall at essentially the same angle of attack.
2) Adjust for different gross weights (RV6a 1650, RV12 1320). Induced drag is proportionate to weight; after all it is the backward vector component to the wing’s lift.
3) Induced Drag (pounds) = (1320/1650) * 764669.98 / [CAS(mph) + 3]^2

Then proceeded to the Parasitic curve.
1) The two aircraft have essentially the same fuselage cross-section and length. The RV12 has a longer wingspan and area. The RV6’s gear was faired while the RV12’s are not. The RV6a is flush riveted while the RV12 uses low profile rivets.
2) The RV12’s best glide speed and therefore the speed where parasitic drag and induced drag should be equal (by definition) is, according to the POH, 98 mph.
3) The coefficient for the parasitic drag curve was adjusted until the parasitic and induced drag at 98 mph were equal.
4) The resulting coefficient for the RV12 (0.00624409) is larger than the RV6a (0.00594791) and steepens the curve, indicating that the RV12 has more parasitic drag than the RV6a. The ratio of the coefficients (RV12/RV6a) is 1.04979. In other words the RV12’s parasitic drag is about 5% greater than the RV6a.
5) RV 12 Parasitic Drag (pounds) = 0.00624409 * CAS(mph)^2

From these curves the following can be determined (maybe):
1) The Total Drag curve, also known as Polar Drag, can be drawn by adding the induced drag and parasitic drag values at each of the airspeeds.
2) Carson’s speed, (optimal cruise) can be determined by finding where a line drawn from the origin will be tangent to the Polar Drag curve. By visual inspection this appears to about 129 mph
3) According to the Café papers:
Thrust Horsepower Required = AS (mph) * Drag (lbs) / 550:
Which at best glide speed (98 mph) is 21.37 THr, at minimum decent speed (min power required) (72.6 mph) is 18.47 THr, and at 135 mph is 35.82 THr.
4) Minimum decent speed is the low point on the Thrust Horsepower required curve. According to the Café papers: Sink rate can be calculated from:
Sink rate (fpm) = drag (lbs) * speed (fpm) / weight.
At the Minimum decent speed of 72.6 mph the calculated sink rate is 677 fpm producing a glide angle of 6.1 degrees.
5) Maximum range glide (best glide speed) is 98 mph (from POH). At 98 mph the sink rate calculates out to a 784 fpm glide, which results in a minimum glide angle of 5.2 degrees.
6) From the RV 12 and RV6a parasite drag curves coefficients, and knowing that the RV6a flat plate area is 2.32 sq ft (Café Report) the calculated equivalent flat plate drag area of the RV12 calculates to 2.436 sq ft. Again about a 5% difference.


The Graph:

[Tried to post without any luck.]


Assistance/corrections from someone who actually understands this stuff would be appreciated.

Can someone add the plot of the Thrust Available from the Rotax and prop used?

Regards, Dave

Whoa! Why is it after reading your post I am afraid to fly now?

The Graphs

Now what does this tell me? Can someone explain? Thanks

Peterk said:

Now what does this tell me? Can someone explain? Thanks

Click to expand...

It's telling you the -12 won't fly.

These lines look like my approaches to final!

Peterk said:

Now what does this tell me? Can someone explain? Thanks

Click to expand...

I think the chart shows the process of building a -12:

- The diagonal blue line represents the amount of money spent.

- The curved blue line represents the amount of work left.

- The green curve represents the builder's enthusiasm level. The dip is when he was working on the longerons.

- The red line represents his excitement level.

There is a way!

Please look at www.n17hh.net and follow either the article link or the PowerPoint link. The article was published in "Experimenter" earlier this year. The presentation was recently developed for the December 3 meeting of EAA Chapter 13 to do a conceptual quick overview of the article.

In a nutshell, I have developed methods for in-flight determination of the drag curve by building on the work of CAFE and of Jack Norris and Andy Bauer as reported in 1995 in Sport Aviation.

I present some old information in new ways and some new ideas about how to get the airplane to reveal it's real performance. The article is accompanied by a spreadsheet which makes many of the calculations either zero work or extremely easy.

Oshkosh 365 offers a forum for discussing the article. So does this forum. I will be happy for your feedback in either place.

You can determine the drag curve for any GA aircraft without depending upon difficult estimates of factors like tip efficiency or frontal area. You can then compare to the 6A, the 9A or whatever you like.

Re: Drag curves

Well, I'd have to sit down with textbooks I haven't used for years since I no longer have access to the computers I used to use at WPAFB but fading memory says there are several assumpyions that aren't quite right.

1 Yes, induced drag is proportional to weight, but it's more affected by the aspect ratio of the wing, wing loading, and what's happening at the wing tip. I haven't looked at a 6 tip, but the -12 tip has a decent approximation of a Hoerner tip (the concave under surface) which helps reduce induced drag, too.

For subsonic airplanes (the -12 is VERY subsonic)
CD= CDp + CDi
Drag coefficient (Drag/reference Area)= parasite Drag Coefficient + induced Drag Coefficient where
CDp- f/S= equivalent parasite drag/ reference area and
CDi=(CL)squared/(pi x Aspect ratio x oswald's effieicncy factor)

the -12 has more wetted area (more wing) and those pop rivets so the parasite drag will be somehat higher than the -6

Induced drag would have to be calculated-- we're flying slower but at a lower weight so CL isn't necessarily the same, our aspect ratio is higher than the -6, and I suspect that Oswald's efficiency factor may be higher for our wing-- at a guess I'd say out CDi is probably 10-15 % lower-- but all of my experieces is with high subsonic and supersonic design so that's just a gut feel.

2) I SERIOUSLY doubt that the -12's best glide speed is 98 mph! Memory says it's about 70 mph for a C-150 and it's 85 MPH for my AA-1 Yankee which has a MUCH higher wing loading (more weight and less wing area). I'd guess that it's between 1.2 and 1.3 time the stall speed, or in the neighrborhood of 65 mph. Did you confuse feet/second with mph? In all lift and drag computations velocity (V) is in feet/second, not mph.

Once I get the bloody thing built it will be interesting to back into the performance curves if Van's haven't posted them by then, but until then I'll spend my time buikding.

Wayne 120241 N143WM (reserved)
Retired Preliminary Design Engineer, WPAFB, 35 years

hmmm

A couple hours with homegrown software I wrote 18 years ago(!) as an undergrad has me very suspicious of the posted CDi curve down low. Vs available thrust, the math doesn't work. It's a great start to the topic, however.

For reference, my SW says my 9A should come down to ~41 KIAS dirty at 1700lbs, before ripping flow off the wings; my graphic post processor isn't working(I'm surprised any of it ran, it was written for a 486 machine ) but it would seem to be ripping at the root, if I'm reading the data right. It also suggests real controllability, not a snap unzip. It was really written for Re <250K(large model aircraft for the SAE competitions), but it's proven flexible - all I know is that I was a lot smarter then than I am now - I blame my 'management lobotomy'.

DOS emulation in Vista sucks. In XP, it works ok, usually. That's what I learned today.

Rick 90432

I think that the math says the CAFE 6A, if it weighed 1320 pounds would have a best L/D speed of 94.8 mph. The spreadsheet I cited earlier can do much more than that, of course. I don't know if the 94.8 number would actually work for the -12. I doubt it. That's why my work depends on actual flight testing.

What is "best glide" for Drag Curve?

yankee-flyer said:

...

2) I SERIOUSLY doubt that the -12's best glide speed is 98 mph! Memory says it's about 70 mph for a C-150 ...

Wayne 120241 N143WM (reserved)
Retired Preliminary Design Engineer, WPAFB, 35 years

Click to expand...

I agree that 98 is not likely to be a good estimate for the -12. But, Cessna gives best glide, engine out, prop windmilling. That won't work for finding the drag curve.

Example:
Cessna says the best glide for the 152 is 69 mph CAS (=IAS at that speed). But, Jack Norris and Andy Bauer (via CAFE) tested a 152 without a prop and found that the speed for minimum drag (best L/D) is 76.3 mph. The drag difference between the two configurations and conditions is 177.2# vs 130.4 #. The drag curve must be based on the 130 pounds at 76.3 mph in order to be accurate.

Unless you want to use a tow-plane and remove the prop you have to use methods such as I described in my article. I'm omitting the ZeroThrust gizmo that Jack Norris developed because most modern engines don't have the necessary fore-aft slop to allow that to work. If the Rotax does, then that's the most accurate approach.